Evolution of endodermal differentiation in the Ecdysozoa. (A-C) Gut formation in C. elegans, D. melanogaster, and P. caudatus. See main text for details and references. Endoderm is in yellow and mesoderm in blue. Red arrowheads indicate the mouth. (D) In the Spiralia, the endoderm forms from the most vegetal blastomeres, which differentiate into the midgut. In Capitella teleta, the mesoderm mostly originates from the blastomeres 3c and 3d. The mouth forms ventrally, anterior to the closed blastopore. (E) In the Deuterostomia, the endoderm forms at the vegetal pole, and is internalized during gastrulation to form the midgut. The mesoderm develops from a ring of cells concentric to the endoderm (blue), and the mouth opens laterally in Branchiostoma floridae. (F-J) A comparison of the gut-related expression domains of bra, cdx, eve, FGF8/17/18, foxA, foxQ2, GATA456 paralogs, gsc, HNF4, NK2.1, otx, and wnt1 indicates the presence of a conserved gut patterning mechanism in the Ecdysozoa. Most of these genes are also related to gut development outside the Ecdysozoa, although their expression domains may appear in alternative gut regions or germ layers. In F, expression of the multiple HNF4 paralogs of C. elegans is not depicted. In F, G, and I, expression of GATA456 paralogs is depicted under the same colored line. Mesoderm is only drawn at the stage of mesoderm specification. The asterisks indicate the vegetal/posterior pole. All embryos are oriented with the anterior/animal pole to the top and ventral to the left, except C. elegans, in which anterior is to the left and ventral to the bottom. Drawings are not to scale.